Abstract VEO-IBD is a rare but fatal disease in infants and young children with limited effective treatment options. Monogenic cases are observed among younger patients and loss-of-function mutations in the interleukin-10 receptor (IL-10R) are highly associated with severe ulcerative colitis in infants. Hematopoietic stem cell transplantation (HSCT) is the only effective treatment for VEO-IBD, yet, this procedure is not well- tolerated in some patients due to conditioning toxicity and a risk of graft-versus-host disease (GVHD). Therefore, alternative treatment options are needed for VEO-IBD in patients. The anti-inflammatory cytokine IL-10, mainly produced by hematopoietic cell types, is critical in maintaining intestinal tolerance by preventing excessive immune responses to commensal bacteria. Given the severity of disease in VEO-IBD patients and the caveats of HSCT we reasoned that gene therapy may be a potential treatment approach. Using a lentiviral vector containing a codon-optimized mouse Il10r? transgene, we restored IL-10R? expression in Il10rb-/- hematopoietic stem and progenitor cells (HSPCs) in a mouse model of spontaneous colitis. Upon lentiviral gene correction, we observed a significant rescue in weight and pathology when compared to mice that received mock vector treatment. These findings provide a strong rationale for further optimization of this gene therapy approach to treat VEO-IBD. While lentiviral vector safety is improving, additional efforts to increase vector specificity are needed to reduce ectopic gene expression, strengthen cell-specific gene expression, and limit the viral titer required for in vivo effectiveness. Moreover, while IL-10R restoration in the hematopoietic compartment ameliorates IBD symptoms, the specific cell types that require IL-10R restoration to promote intestinal homeostasis remains largely unexplored. Strong evidence exists for the requirement of IL-10R in both T cells and macrophages (M?s) for intestinal homeostasis. The research proposed here will test the hypothesis that M?s and T cells play complementary and non-redundant roles in IBD pathogenesis due to defective IL-10R signaling; thus lineage-specific restoration of IL-10R expression in both lineages will be necessary to effectively prevent IBD. Preliminary data demonstrate adequate vector performance in gene correction and treatment of colitis in an Il10rb-/- mouse model. To further refine this vector system, transplantation of transduced HSPCs will be performed to test the ability of cell-specific vectors to a) restrict transgene expression in specific lineages; b) restore IL-10R signaling; c) promote stable IL-10R expression over time. Gene correction of Il10rb-/- HSPCs using cell-specific vectors or, alternatively, a reliable ubiquitous lentiviral vector, will be performed in mouse model of colitis to test which specific hematopoietic cell types require gene correction to treat disease. The results of these studies will not only further elucidate the biological relevance of each pathway in pathobiology of IBD, but could also determine a future translational cellular product to be utilized in therapeutic interventions.